Boron carbide, used as a neutron absorber, undergoes nuclear reactions producing relevant quantities of He. The understanding of He kinetics at the atomic scale in the material is still in its infancy, in spite of decades of experimental work devoted to the characterization of He containing, irradiated, boron carbide samples. The interplay of He itself with intrinsic defects created by irradiation on kinetics is still almost completely unknown.

In this paper we present an exhaustive study of vacancies and substitutional helium impurities in boron carbide using Density Functional Theory. Analyzing the stability and mobility of these defects allows us to consider diffusion mechanisms other than the known interstitial mechanisms.

We find that vacancies trap He interstitials, raising the activation energy of 2D diffusion to approximately 2 eV. The trapping mechanism is different according to the charge state of the vacancy: in p-type conditions, when vacancies are neutral or positive, He diffuses via a dissociative mechanism and is trapped in a substitutional position; in n-type conditions, negative vacancies trap He atoms traveling in an adjacent $\left\{111\right\}$ plane by a charge transfer driven distorsion.

No favorable vacancy assisted diffusion mechanism was identified for substitutional He atoms, except the dissociative one previously mentioned. Other possible vacancy diffusion mechanisms, which we also analyzed, are hindered by the high activation energy of vacancy self-diffusion.

用作中子吸收剂的碳化硼经历核反应，产生大量的He。尽管数十年来的实验工作致力于表征含氦，辐照的碳化硼样品，但对材料中原子级的He动力学的理解仍处于起步阶段。He本身与动力学辐照产生的固有缺陷之间的相互作用仍然几乎是完全未知的。

在本文中，我们使用密度泛函理论对碳化硼中的空位和取代氦杂质进行了详尽的研究。分析这些缺陷的稳定性和迁移率使我们可以考虑除已知的间隙机制以外的扩散机制。

我们发现空位会捕获He间隙，从而将2D扩散的激活能提高到大约2 eV。根据空位的电荷状态，俘获机理是不同的：在p型条件下，当空位是中性或正电荷时，He通过解离机理扩散并被俘获在取代位置；在n型条件下，负空位会俘获在相邻原子中传播的He原子$\left\{111\right\}$ 平面由电荷转移驱动畸变。

除先前提到的离解性原子外，没有发现有利的空位辅助扩散机制可用于取代的He原子。我们还分析了其他可能的空位扩散机制，这些空位扩散机制是由空位自我扩散的高活化能所阻碍的。